This alloy relates to an expansion-controlled light alloy piston for internal combustion engines, particularly for diesel engines, comprising at least one plate-like or strip-like expansion control element, which preferably consists of steel and is bonded to the inside peripheral surface of the carrying portions of the skirt of the piston and is embedded in the cast hubs of the piston pin eyes, and which on that portion which is adjacent to the piston head is covered on the inside by an embedding inner bead, which extends between the hubs of the piston pin eyes to a level which is spaced above the lower end of the expansion control element by 10 to 50% of the height of said element.
Such an expansion-controlled light alloy piston is known in its basic form as a segment strip piston and has been described in German Patent Application No. 10 78 387. The expansion control action of such control elements is due to the fact that the light alloy layer which lies on the outside peripheral surface of such control elements shrinks onto the expansion control element a said layer cools after the piston has been cast and said shrinkage gives rise to tensile stresses in the light alloy casting and to compressive stresses in the expansion control element. In those embodiments in which the top edge of the expansion control element extends into the oil ring groove, that element will be exposed as the oil ring groove is machined so that the initial stress equilibrium between the expansion control element and the embedding light alloy casting is disturbed and the expansion control element snaps outwardly and increases the inherent tensile stresses in the embedding light alloy-casting. This improves the expansion control action. A gap of about 50 .mu.m is formed between the expansion control element and the embedding cast inner bead extending between the hubs of the piston pin eyes. When the temperature of such piston rises during the operation of the engine, the stresses will decrease in dependence on the temperature rise so that the elastic deformation of the embedding light alloy casting decreases. That action is superposed on the natural tendency of the part concerned to expand when heated so that the parts will exhibit a radial thermal expansion as though they consisted of a single material having a coefficient of expansion between the respective coefficients of expansion of the piston material and the material of the expansion control element. Such pistons exhibit a good expansion control action particularly adjacent to the expansion control element but have only a slight expansion control action on the lower skirt portion of the piston.
In other pistons, known from German Patent Specifications No. 17 50 426 and 12 71 271, the expansion control action is improved in that strip-like or segment-like expansion control elements provided in the upper skirt portion are combined with expansion control elements consisting of sleeve segments and disposed in the lower skirt portion. The expansion control action of such pistons is due to two basically different principles. In the upper skirt portion the expansion control is mainly the result of the above-described shrinkage, as described hereinbefore, whereas in the lower skirt portion the expansion control is mainly due to a bimetal effect. The expansion-controlling sleeve segments disposed in the middle and lower skirt portions and the light alloy layer lying on their outside peripheral surface constitute bimetallic expansion control elements, which is the case of a temperature rise tend to change their curvature. As a result, the skirt is deformed to an oval shape in such a manner that the largest radial expansion of heat takes place in the axial direction of the piston pin and the dimension in the plane of oscillation of the connecting rod is correspondingly reduced. That design, which is adopted to provide for an expansion control action throughout the piston skirt, involves mainly the disadvantage that the expansion control elements have a large area so that the piston weight does not meet the requirements for pistons in internal combustion engines. Besides, expansion control elements having a large area are undesirable from the aspect of casting technology.
In order to meet the more stringent requirements regarding reduction of noise and reduction of friction losses, the piston should be light in weight, have a small assembling clearance, an axial contour having only a minimum curvature and the smallest possible thermal expansion in excess of that of the inside surface of the cylinder liner. To ensure a maximum life with unchanged riding comfort, the piston must be stiff particularly at its skirt end so that the skirt will not be deformed to a substantial extent.
A higher stiffness of the lower skirt portion has an undesirable influence particularly on the expansion control action of pistons which exert an expansion control that is due to shrinkage. In such pistons there will be only a small expansion control action or no expansion control action at all in the lower skirt portion, particularly if there is a large distance between the measuring point at the lower skirt portion and the lower edge of the expansion control plate. Pistons in which an expansion control action is exerted as a result of shrinkage and by bimetal action do not meet the requirements for a light weight.